The present invention concerns a liquid crystal display device, display screens based on this structure, and a method for making such display screens.
The dramatic growth of the liquid crystal display (LCD) market has been mainly fueled by the strong demand for flat and lightweight color displays in notebook computers. High quality LCD displays require an active driving of pixel electrodes with thin film transistors (TFT) which are, however, costly to produce. This prevented so far spreading of LCD displays in the monitor and television market.
A display screen is an electro optical device to make data or images appear on a monitor or on a front end device. Most display screens work with the same principle as today""s television screens using a cathode ray tube (CRT). Flat panel liquid crystal displays (LCD) are thin display screens that are used in a broad range of fixed and mobile devices like appliances, mobile phones, CD players, personal digital assistants and portable computers because they have a smaller geometric form factor and lower weight. More recently the brilliance, color saturation and pixel resolution of LCD displays have been improved so that they start to challenge CRT and other types of displays used in desktop monitors and television sets.
Nearly all modern flat panel displays use LCD technologies. LCDs utilize two sheets of polarizing material with a liquid crystal (LC) solution between them. An electrical field applied between electrodes across the liquid causes the crystals to align vertically so that the polarizing plane of the light is no longer rotated by 90 degrees and thus prevents the light from passing through the crossed polarizing sheets. Each cell, defined by the top and bottom electrode therefor, is like a shutter or valve either allowing light to pass through or blocking the light. Liquid crystals generally assume a restricted set of orientations with respect to a surface. This phenomenon, called anchoring, is the result of orientation dependent interactions between the surface and the liquid crystal. The control of the orientation in LCDs is central to the operation of all displays. A substantial amount of effort has been directed to deploy reproducible methods. Rubbing of a polyimide sheet is one known approach being employed to control the orientation.
Self assembled monolayers (SAM) of linear alkanethiols on metals like gold (Au), silver (Ag), and copper (Cu) or of linear phosphoric acid molecules on conductor oxides, e.g. nickel oxide, confer a remarkable level of control over structure and chemical functionality of their surface. In a publication entitled xe2x80x9cOrientations of Liquid Crystals on Self assembled Monolayers Formed from Alkanethiols on Goldxe2x80x9d in ACS Symp. 695, pp. 81 102 (1998), by Abbott et al. it is shown that unlike pure conductor surfaces, SAM covered surfaces direct liquid crystals to a planar anchoring, for SAMs formed from molecules longer than twelve carbon atoms. Azimutal ordering of the liquid crystal domains is introduced by formation of SAMs on Au films that are created by oblique or tilted evaporation. An external orientation parameter, the oblique evaporation, is needed to turn the system into a macroscopic light valve for display devices. The azimutal direction of liquid crystal alignment is perpendicular to the evaporation direction for SAMs with even numbers of carbons in their chain and parallel for SAMs with odd numbers of carbons in their chain. The mechanism for the alignment is the preferential roughness created by the oblique evaporation, a result also apparent from xe2x80x9cAlignment of liquid crystal on a Polarizing Metal Filmxe2x80x9d in Appl. Phys. Lett. 56, pp. 1723 1724 (1990) by D. Armitage. Metal layers in both referenced papers are only partially transparent thus precluding manufacturing of commercially useful displays with high brightness.
Switching of one liquid crystal cell from the planar anchored status, i.e. where the plane of polarization is rotated by 90 degrees to the homeotropic status, i.e. no rotation, uses the application of an electrical field, as stated above. This can also be extended to an array of cells as it is found in a simple small monochrome display. In such a case each pixel is addressed for a short time only by a potential difference applied to its column and row. This passive matrix approach is simple, but due to the limited time available to address each pixel it creates visible artifacts and cannot be used to drive large high quality arrays.
Large LCD arrays are better controlled by an active matrix approach. Here each pixel is driven by a thin film transistor (TFT) that charges the pixel capacitor during the time it is switched on and isolates charge during the time other pixels are addressed. This active matrix display produces images with higher quality and is also capable to drive the threefold larger arrays needed for color images. In fact, the image quality of active matrix TFT LCD color displays can easily compete with CRT displays because refresh rates are higher. The TFT technology provides the best resolution of all flat panel techniques but is also the most expensive.
Color liquid crystal displays consist of several hundred thousands to millions of pixels, and each pixel consists of three sub pixels that separately control R (red), G (green), and B (blue). A color filter is used for each sub pixel in order to display RGB, so a full color image can be obtained by combining such sub pixel displays. In such a case, only one third of the light can be utilized. Color can also be generated by separating white light into R, G, and B by a prism or a grating. To get a full color image each color is then gated separately by a subset of a pixel triplet. In SID Symposium 1998, p. 199, a method is proposed wherein, as a liquid crystal projector, arc lamp light is reflected on a diffraction grating surface to be separated into RGB light, collected by a microlens array and have its transparency controlled by a liquid crystal layer for each sub pixel.
LCD screens use backlight so that they are visible in the dark and show a higher contrast in bright environments. Space requirements of the backlight in portable displays are tight so that no far field projection is used. Instead, typical backlights are composed of a linear light source at the side of the display that feeds light into a planar light guide that illuminates the whole display area homogeneously. Backlights used to date emit light with a large opening angle which is welcome to provide a good viewing angle for the user but prevents use of the light separation protocol described above. In the Japanese patent application Nr. 011 43246, of Feb 2, 1999 entitled xe2x80x9cTransparent Type Liquid Crystal Displayxe2x80x9d, a type of substrate is described that creates parallel light which is separated by means of a grating into RGB light. R, G, and B are emitted under slightly different angles onto a microlens array and focused onto the respective LCD pixels. Having passed a first polarizer sheet, the liquid crystal layer, and a second polarizer the light needs to be diffused again to create a homogenous angle independent color for the viewer.
The fabrication costs of displays are directly proportional to the number of the lithographic layers, the number of vacuum deposition steps, and the overall number of assembly steps. A passive matrix display needs assembly of two crossed polarizers, two transparent plates with patterned ITO electrodes possibly with an additional conductor black matrix, two rubbed layers of polyimide, and when color is needed an additional sheet containing patterned RGB color filters. For active matrix displays the fabrication costs of TFTs currently push up the overall cost by a large factor, because TFTs are fabricated using a four to seven mask process and either a Co or an Al gate metallization.
In a conventional LCD, the liquid crystal material is placed in a homogeneous electric field between the two transparent electrodes. The light propagates from a backlight source through a bottom transparent plate, a first transparent electrode made of indium tin oxide (ITO), an insulating layer, the liquid crystal molecules, a top electrode and a top transparent plate. Manufacturing of the transparent ITO bottom electrode requires one lithography step and one vacuum step. ITO is the material of choice for electrodes because it is one of the few transparent conducting materials that can be processed technologically. Typically, pixel electrodes are made from 50 nm thick ITO. At this thickness the ITO absorbs between 6 and 16% of the incident light. Metals, and copper in particular, although they can efficiently conduct currents, are not adequate for transparent electrodes because they strongly absorb the light much before a conducting layer is realized.
Metal meshes with dimensions of the order of several microns have already been suggested as highly conductive structures to reduce the relative high resistance of a transparent conductor like ITO. Examples are described in U.S. Pat. Nos. 5,293,564, 5,455,899, and 5,131,065. Their strong optical diffraction, however, precludes their use in useful display devices. Gratings or other structures with a regular pitch of the order of the wavelength, i.e. 0.1 20 microns, typically show strong diffraction. The patent application WO 99/59024 xe2x80x9cDisplay Substrate Electrodes with Auxiliary Metal Layers for Enhanced Conductivityxe2x80x9d describes several processes for self aligned deposition of additional conductor layers onto ITO to improve the overall conduction. The first two preferred embodiments show how conductor wires can be added laterally to ITO areas. A third and fourth embodiment describe how periodic arrays of holes, triangles, squares, and hexagons, can be used to define electrodes with improved conductivity. The analysis concludes with an assessment of the percentage of light transmitted being proportional to the area density of the openings.
Transparent electrodes made from metals can be made by use of a mesh of wires, as described in EP 0 969 517 A1. The disadvantage of meshes is their strong tendency for light scattering and their visibility to the human eye. This is the main reason why such mesh electrodes have not been used for LCDs so far. When the mesh is made finer and finer, the pattern eventually becomes invisible to the viewer""s eye. Such a mesh electrode will then be perceived as a partially transparent surface with the ratio of conductor to the ratio of glass determining the transmission ratio.
The size of the conductor mesh described in WO 99/59024 is relatively large because the uniformity of the electric field across the LCD pixel is guaranteed by the presence of the ITO in the openings. Elimination of the ITO while maintaining the homogeneity of the electric field requires a much smaller size of the arrays with holes being smaller than one or two micrometers. It is known from optics, that periodic arrays of structures diffract light and create intensively colored higher order beams that disturb the normal operation of the display, i.e. white light is separated into the rainbow colors and different colors are seen from the same pixel under different angles of observation. Diffractive phenomena are observed for structures that are periodic within the coherence length of light, typically 10 microns for white light that underwent several reflections in a backlight.
Transmission ratios of 80 90% require the lines forming the mesh to be at least 10 times smaller than their repeat distance. Both the resolution of the human eye and the field homogeneity dictate repeat distances below 1 micrometer. This results in a line width of the order below 100 nm. Traditionally, lithography used for display fabrication can reproduce patterns of the order of 10 microns with an overlay accuracy of the order of 1 mm. Currently, the lithography costs for LCD manufacturing at sub micron scales are not affordable because of a lack of low cost patterning methods that can reproduce such dimensions.
Usually color separators or color filters sit below the LCD color glass plate which means that the separator is in contact with the liquid crystal (liquid crystal) fluid. It is a disadvantage of this known approach that the conventional color filters have to be passivated such that they do not contaminate the liquid crystal. The conventional color filters are usually made by dyeing, pigment dispersion, printing or electro deposition, as described in the article xe2x80x9cColor filter technology for liquid crystal displaysxe2x80x9d, R. W. Sabnis, Displays 20, 1999, pp. 119 129.
A basic lithographical method is described in xe2x80x9cPrinting Technologyxe2x80x9d 4th ed. Delamare Publishers, Albany, 1996. Details about novel applications are for instance given in the publication xe2x80x9cSoft Lithographyxe2x80x9d, by Younan Xia and G. M. Whitesides, Angew. Chem, Int. Ed, 1998, 37, pp. 550 575 and in the U.S. Pat. Nos. 5,512,131, 5,900,160, 6,048,623, and 6,060,121. Other US patents that describe soft lithography are: U.S. Pat. Nos. 5,669,303, 5,725,788, and 5,727,977.
High resolution printing is an alternative approach to conventional lithography. Several parameters important for accurate and defect less patterns have been identified and controlled, as described in xe2x80x9cTransport Mechanisms of Alkanethiols during Microcontact Printing on Goldxe2x80x9d, E. Delamarche et al., The Journal of Physical Chemistry B, Vol. 102, No. 18, pp. 3324 3334. Issues that are being studied to be able to replicate arbitrary patterns of small and large dimensions are polymer mechanics and overlay accuracy of the stamp, addressed by H. Schmid and B. Michel in the article xe2x80x9cSiloxane Polymers for High Resolution, High Accuracy Soft Lithographyxe2x80x9d, Macromolecules, Vol. 33, No. 8, pp. 3042 3049 and by Bietsch and Michel, J. Appl. Phys., 88, pp. 4310 4318 (2000). Ink diffusion is another issue that is dealt with by E. Delamarche et al. in the above mentioned paper. The surface chemistry to allow easy separation of stamps from master and substrate is described by H. Biebuyck et al. in xe2x80x9cLithography beyond light: Microcontact printing with monolayer resistsxe2x80x9d, IBM J. Res. Develop., Vol. 41, No. 1/2, January/March 1997, pp. 159 169. Recently, microcontact printing has been extended to chemically amplified printing where a catalytically active molecule is printed onto a substrate, as described by H. Kind, M. Geissler, H. Schmid, B. Michel, K. Kern, and E. Delamarche in xe2x80x9cPatterned Electroless Deposition of Copper by Microcontact Printing Palladium (II) Complexes on Titanium Covered Surfacesxe2x80x9d, Langmuir 16 (16), p. 6373 (2000).
The theory of Mie explains the scattering of a planar monochromatic wave by a homogeneous sphere of any diameter and of any composition situated in a homogeneous medium. An equivalent solution exists from Debye. Simple calculations using the Huygens Kirchhoff theory allow the prediction of light intensity of individual scatterers as function of viewing angle (theta) and as function of the size of a scatterer. Scattering of light by long circular cylinders has been studied by Seitz and Ignatowsky (Ann. d. Phys. (4), 16, 746 (1905); ibid., 19, p. 554 (1906); Ann. d. Physik (4), 18, p. 495 (1905)) and Schaeffer and Grossmann (Ann. d. Phys. (4), 31, p. 455 (1910)): the formulas are similar to those of Mie relating to the sphere for the scattering circle perpendicular to the axis of the cylinder. Diffraction or scattering can be extended from a single sphere or cylinder to a system with any number of objects provided they are randomly distributed within the coherence length of the light used and separated from each other by distances that are large compared to the wavelength.
In U.S. Pat. No. 5,925,259 a process for producing lithographic features in a substrate layer is described, comprising the steps of lowering a stamp carrying a reactant onto a substrate, confining the subsequent reaction to the desired pattern, lifting the stamp and removing the debris of the reaction from the substrate. Preferably, the stamp carries the pattern to be etched or depressions corresponding to such a pattern. Using these methods, patterns with submicron features can be generated.
A beneficial effect that comes into play when mesh openings are smaller than the wavelength of the light is the forward scattering effect, as addressed in the above mentioned European patent application EP 0 969 517 A1.
It is a feature of the present invention to provide a new display structure.
It is another feature of the present invention to provide new displays based on this display structure.
It is yet another feature of the present invention to provide a method for making such new displays.
The present invention provides a liquid crystal display device, a method for making such a liquid crystal display device, and a display device. Furthermore, the present invention provides a conductor mesh for use in displays.
One embodiment of a liquid crystal display device includes a grating layer, a transparent substrate layer, a pixel electrode, and an active circuit element layer with a field effect transistor. The liquid crystal display device further includes a liquid crystal layer, a counter electrode, and a transparent cover plate. Light emitted by a light source travels through the liquid crystal display device.